Testing H745 Regulators

Brent Hilpert bhilpert at shaw.ca
Wed Jan 26 21:33:18 CST 2022

On 2022-Jan-26, at 3:41 PM, Rob Jarratt via cctalk wrote:
> I am trying to test a couple of H745 regulators with a DC bench PSU and I am
> having some problems with testing them.
> My bench PSU is a twin unit so I can supply the +15V required as well as the
> "AC" input using 20VDC from the other half of the bench PSU. The problem is
> that I don't think the bench PSU can supply enough startup current to allow
> the regulator to run. It can only supply 5A max.
> I have seen with the H744s that if I put too big a load on them, then they
> can't start because of the heavy startup current required. I can start them
> with a lower load and then add load once the regulator is running without
> breaching the current limit of the PSU.
> With the H745s I have tried reducing the load to see if I can get them to
> start, but a 10R load appears to be too much and the regulators draw the
> full 5A without outputting -15V.
> I have two H745s, both exhibit the same behaviour. I suppose they could both
> have the same fault, but I am inclined to think that perhaps they need a
> higher startup current than I can supply. Can anyone confirm this?

20V on a 10 ohm load: current = 2A.
15V, 1.5A.

In this regulator design there is no path for more current than that which the load draws, aside from temporary peak currents to charge capacitors. If you're drawing 5A DC from the bench supply, something beyond 'failure to start' is wrong. I would expect this supply to operate at small load regardless.

What is happening to the bench supply voltage? Does it go into current limit? Does this bench supply have an adjustable current limit?, so that you could run it up starting at a low current while taking measurements. Or, does the current respond with some linearity to varying the input voltage?

What happens with no load R?

Are you running it for any length of time at 5A? (Sounds like a bad idea at this point) Anything getting warm?

Is the 723 socketed? Pull it and run it up while watching what happens around the drive transistors and elsewhere.
If the 723 is not socketed, consider pulling Q5 or opening it's emitter connection. With no drive to the drive transistors, input current should be nil.

Are any of the drive transistors socketed, so they could be measured out of circuit? and other R measurements made without them in circuit?

Pull F1 to isolate circuitry. Still draws current?

Have you looked for shorts/leaks?, especially leaky junctions in transistors Q2::Q5.
e.g. R measurements, no F1, no load R, both directions:
Q2.B-C ?
Q2.E-GND ?
Q2.C-GND ?
-15-GND ?
Settling time for cap charge/discharge may be needed.

In answer to your earlier question, no, the +15V is not the reference, it is the supply for the 723 regulator IC. The reference is the internal reference provided by the 723, though that internal reference is powered inside the IC from the +15V.

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